Using various strains of inbred mice with high and low leukemia incidence we seek to define the interaction of murine cytomegalovirus (MCMV) with leukemia virus (MuLV) by studying the transformation of lymphoid cells, the activation of latent or endogenous virus and leukemogenesis. We intend to explore the co-carcinogenic effect of MCMV on malignant transformation of salivary and prostate gland in vivo and in vitro. We seek to define the genetic loci that control the susceptibility to MCMV and to compare this, or find the relationship if any between the regulation of murine response to this virus, to the virus of MuLV, and the expression of leukemia.